SEISMIC INTERPRETATION OF THE WYOMING OVERTHRUST BELT W.D. Williams and J.S. Dixon

Champlin Petroleum Co.1

INTRODUCTION Significance of Seismic Line

This seismic line provides a regional presentation of thrustbelt tectonics and demonstrates the structural style of the prolific producing trend within the Wyoming salient of the Western Over- thrust Belt. The line traverses the Whimey Canyon and Ryckman Creek fields, the Absarkoa and Darby plates, and terminates over Green River Basin strata (Fig. 1).

The geometry of the thrust faults can be seen on Figures 2 thru 5. Beginning at the west end of the line, the Absaroka Thrust overrides the Darby Plate sedimentary rocks and then subcrops in a position directly above the location of the trailing edge of the Darby Thrust Fault detachment, which lies within the Cambrian Gros Ventre Shale (Figs. 3,5) . The Darby Thrust begins at the decollement, ramps up over Green River Basin strata, and subcrops beneath a Tertiary section in a series of imbricated faults (Figs. 3,5).

The fold geometries within the Absaroka Plate show an immediate visual contrast (Fig. 3) with the lower amplitude folding along the leading edge of the Darby. Also easily recognizable is the uncon- formity that exists in the shallow part of the line, which separates Tertiary from Mesozoic sedimentary rocks. The east end of the seismic line is significant in that it shows the effect of the last major thrust fault, the Darby, terminating against Green River Basin strata. This compression caused an over- thickening of the Upper Cretaceous Hilliard Shale because of lateral piercement beneath, and in front of, the thrust. The basinal rocks to the east are otherwise undisturbed and show regional west dip.

Location of Seismic

in an east-west direction, parallel to structural dip and normal to the strike of the major thrusts.

REGIONAL GEOLOGY

The line is located in Uinta County, Wyoming, (Fig. 1) within the Overthrust Belt and is oriented

Tectonic Setting The Wyoming Salient of the western Overthrust Belt should be considered relative to known

western North American tectonic history, particularly accretionary tectonics and the original western cratonic edge. The western part of the province at the Crawford Plate is about 280 mi (450 km) east of the strontium 0.706 ratio line (Fig. 7), well within the bounds of the original cratonic edge. It now appears so isolated from the known accretionary terranes of the Blue Mountains/Seven Devils, Roberts Mountain, and Sonoma terranes by intervening Basin and Range tectonism that the direct mechanistic linkage of overthrust and accretion to the craton margin is obscure.

piston-like overthrust geometries are impossible because the distance of force transmission far

1 A wholly owned subsidiary of Union Pacific Corporation

Rock strength and mechanics studies indicate another severe difficulty: the collision and resultant

exceeds the rocks capability to transmit that force. A relief from this dilemma is provided by under- thrust concepts (Misch, 1960; Coney, 1973; Burchfiel and Davis, 1975; Dickinson, 1976; Lowell, 1977) in which there is no transmission of force from accretionary collisions in the west. The force transmission was from the immensely stronger cratonic side during mid-Atlantic spreading. With this setting in mind, the upper plates of overthrust sheets merely stood in place and were deformed u p ward as the lower, undeformed autochthonous block was isostatically depressed and moved westward. According to this concept, initial deformation by overthrusting was significantly closer to the western cratonic edge, and the cratonic edge was defined by strontium ratio line moved westward both literally and relatively through time.

concerning the role of gravity sliding, basement involvement, and internal features on a far smaller scale.

the easternmost three thrust systems (Crawford, Absaroka, and HogsbacWDarby). This definition has been started and may be ultimately achieved by increased drilling and the acquisition of more and higher quality seismic control. The cratonic ramp (Fig. 1)) onto which the Wyoming Salient has been developed is already adequately defined by regional seismic lines as a gentle, westward-dipping plane (300 fdmi or 57dkm) that has in places been deformed into broad arches such as the Moxa Arch (Dixon, 1982). The mode of this deformation was apparently by isostatic loading of Cretaceous fore- deep or foretrench sedimentation (Jordan, 198 1).

The regional basal sole or detachment is well defined by seismic. It is located within the Cambrian Gros Ventre Shale, a middle unit of the transgressive Cambrian Flathead, Gros Ventre, Gallatin sequence (Fig. 8) which thickens westward, as do all other Paleozoic units, and deposition very closely paralleled the crystalline Precambrian basement surface of very low relief. In the northern part of the province and certainly in Montana, Paleozoic rocks are separated from crystalline basement by a west- ward-thickening low grade metamorphic Precambrian sedimentary sequence correlative with the Belt Group or Uinta Group. Where these metasedimentary rocks are present, there is some seismic indication that the regional detachment can occur stratigraphically lower within it, but never into crystalline basement (Dixon, 1982).

Depositional Setting. The stratigraphic sequence of mainly Paleozoic and Mesozoic rocks is about 40,000 ft (12,200 m)

thick (Fig. 8). The carbonate Paleozoic rocks are very competent, while sandstones, shales, carbonates and evaporites of Mesozoic age are rather incompetent, causing styles of brittle and plastic deformation to develop. This stratigraphic sequence provides numerous reservoirs and source rocks, but by far the most significant source rocks are in the Lower Cretaceous sequence.

Paleozoic deposition older than the Pennsylvanian Weber Sandstone was all thickening westward and represented rather normal marine cratonic shelf-to-edge sedimentation with numerous discon-

Problems of the grand-scale geologic setting will continue to be argued for a long time, especially

There is hope in the near future to achieve a very defendable definition of internal geometries of

LOCATION MAP Figure 1

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YOMING

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SEISMIC PARAMETERS Champlin Petroleum Company

220ft. Processing Client / Amoco Contractor Group Int. Year

Dynamite Spread 517e-1 l (F l lw5170 Sample Rate 2ms- Source Recording DFS

Charge Size Final Filter 9/18-36/66 System 501bs-

Final Scaling 500ms- AGC Channels 48 Hole Depth 140ft- CDP Fold r 8 Special Processing Instrument dephase, time

variant filter, spectral whitening, surface consistent statics, migration

660ft. Shot Int. 1972

I 1 13 Wyoming Overthrust Belt Williams and Dixon I

Seismic Exploration of the Rocky Mountain Region, 1985

2009 Rocky Mountain Association of Geologists & Denver Geophysical Society

ELEVATION IN F-EET (THOUSANDS) F A

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Wyoming Overthrust Belt 16 Williams and Dixon

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KILOMETERS DIP ANGLE LINE 1 - INTERPRETED LEGEND 0 I

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8 FOLD - SINGLE HOLE DYNAMITE - 1972 MIGRATED TIME SECTION a 0. 0 1 2

P - Paleozoic C - Cambrian Gros Ventre

J - Jurassic Jn - Jurassic Nugget

PROCESSED*BY: CHAMPLIN PETROLEUM CO., 1984 STATUTE MILES APPROXIMATE VERTICAL EXAGGERATION

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HITNEY CANYON ' 7 7

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8 $BEssiE BTMS '83 / I LAND GRANT

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Figure 7: Fields and year of discovery in the Wyoming Salient Figure 8: Generalized stratigraphic column of the southern portion of the Wyoming Salient. Productive intervals and potential source rocks are indicated.

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-- - -- L WESTERN U. S. SELECTED

MEXICO rECTONlC ELEMENTS Figure 9.

AMOCO AMOCO 1-CPC-4 13A MARATHON 1 -CPC-549A

3-17N-116W 21-17N-115W PROJ. 2.1 MI. TO SOUTH PROJ. 1400' TO NORTH

1 ALBERT CRK 36- 18N-117W

PROJ. 3.1 MI. TO SOUTH WHITNEY CANYON FIELD RYCKMAN CREEK FIELD

CHEVRON AMOCO HAMILTON AMOCO 2 1-30E 1-CPC-224 HAMILTON FED 1-8 1-CPC-549B

8-17N-116W 1-17N-116W 30-17N-119W 19-17N-118W

25,000' 20 15 10 5 0 Figure 10

I Williams and Dixon Wyoming Overthrust Belt 2o I

formities within basically parallel deposition. Localized Pennsylvanian tectonism disrupted the normal carbonate patterns.

By Permian time the cratonic edge was a complex strike-slip margin with generally open marine conditions to the west and topographically separated subbasins, causing more complicated facie pattern in the Phosphoria Formation. (Tisoncik, in press). This disruption of the cratonic edge may have been caused by the accretion of exotic western terranes such as the Stikinia, Cache Creek, and Shuswap. Effects from the accretion of the Sonoman and Roberts Mountain terranes of Nevada are not in evidence. These concepts are very speculative, but there is little doubt that possible terrane accretion by right-lateral translation did seal the area from normal, western open marine conditions by Triassic time.

In Triassic time, arid nonmarine to shallow marine conditions prevailed, culminating with regional dune deposition of the Nugget Formation and anhydritdsalt deposition later in the Jurassic. Early Sevier mountain building formed an orographic barrier to moisture-providing westerly winds at these latitudes. Isostatic loading of the craton by eastward (relative) advancing thrust systems caused the development of the Cretaceous Rocky Mountain geosyncline or foretrench (Jordan, 198 l), and resulted in thick marine Cretaceous deposition between the Sevier front and the cratonic carbonate platforms in the east.

Many of the Cretaceous rocks involved within the Wyoming salient thrusts are synorogenic, proximal clastics that are not of source rock quality, particularly in the western systems. Where thrusts advanced over deeper, Cretaceous marine sequences, there was increased maturation in subthrust source rocks. Angevine and Turcotte (1983) have modeled this maturation for various thicknesses of allochthonous warm plates and concluded that maturation of the Cretaceous source rocks followed thrust emplacement and that migration was of a short distance into upper plate structural reservoirs. The Cretaceous source beds are the main source for upper plate hydrocarbon accumulations (Warner, 1980).

Tertiary fluvial and lacustrine deposition occurred both in front of the Hogsback and Prospect systems as well as in listric normal fault systems of the thrusted belt. Thicknesses up to 10,000 ft (3030 m) are found in some of these half-graben features. Even greater thicknesses were deposited in the Bear Lake area where strong basin and range tectonism caused more significant normal fault displacements.

HISTORY OF PETROLEUM EXPLORATION The Overthrust Belt of western North America has had very sporadic petroleum exploration and

only two segments of these efforts have provided success. The Canadian Foothills successful exploration began with the Turner Valley discovery in 1913, and Wyoming overthrust success began with Pine- view Field in 1975 (Petroleum Exploration, 1981). The Wyoming salient with 20 fields at this time has large reserves (Powers, 1983) which are not yet firmly established because of geologic complexity and continuing activity. Rocks of every geologic epoch from Ordovician to Cretaceous have the capability to contain oil; however, the Jurassic Nugget and the Mississippian Madison formations are volumetrically the most significant. Years of frustration were experienced by overthrust before the discovery of Pineview Field, and the only production was from shallow Cretaceous rocks of the Fossil sub-basin; exploration was then related to surface geology and oil seeps. Dlfficulty in obtaining usable seismic data and interpreting the complex disharmonic structural relationships hindered resolu-

tion at the Nugget Sandstone level. Improvement of seismic data and better regional seismic coverage has resulted in the successful drilling of numerous anticlinal closures concealed beneath irregular Tertiary lacustrine basins and complexly detached Cretaceous and Jurassic units. Figure 9 shows the fields of the Overthrust Belt with their year of discovery. One to five discoveries have been made each year since 1975.

One regional production characteristic is the predominance of sour gas in the western Absaroka trend and the more oil-rich sweet production of the eastern Absaroka trend. Common to both are oil- and gas-prone Lower Cretaceous source rocks in the sub-Absaroka block. Therefore, it appears that reservoir modification of the hydrocarbons is the cause of this sourhweet relationship.

SEISMIC INTERPRFiTATION Key Reflectors

the Jurassic Nugget and the Cambrian Gros Ventre. It should be noted that the Nugget Formation is a very poor reflector and is being identified only because of its prolific producing capability. Most reflections identified on seismic as the Nugget Formation are probably members of the overlying Twin Creek Formation.

The seismic correlation chart (Fig. 6) provides a detailed display of formations and their reflection characteristics. Generally, the events which are most easily mapped on thrustbelt seismic are the base of the Tertiary rocks (unconformity), the Jurassic Twin Creek Formation, the Triassic Ankareh Formation, the Permian Phosphoria Formation (at lower frequencies the large amplitude is probably due to the tuning effect of the Triassic Dinwoody Formation), and the Cambrian Gros Ventre Formation. Reflections can also occur, however, along fault planes. An example of this can be seen along the Absaroka Thrust beneath Whitney Canyon Field and between the Whitney Canyon and Ryckman Creek fields (Figs. 3,5), where the thrust is a strong reflector with Cretaceous events angularly truncated beneath it.

gross packages, rather than attempting a detailed interpretation which could not be presented at this scale. This is particularly true on the leading edge of the Darby Thrust where no attempt was made to identify every fault within the zone of imbrication. Also, rather than forcing an interpretation across zones of no data retrieval, discontinuities in reflections were honored, leaving the complete inter- pretation to the geologic cross section (Fig. 10).

WelI Control Seven wells were used in the interpretation of the seismic line and are identified on Figures 3

and 5. Those wells that are more than 1,500 ft (457 m) off the line are identified by a dashed borehole signifying that their tops were projected. The remaining wells were treated as being on the line and provided a direct tie into the seismic section.

The well selected for the seismic correlaton correlation chart in Figure 3 is actually projected in from the Church Buttes Field, nine mi (1 4.5 km) away. This well, the Church Buttes Unit Well No. 19 (sec. 8, T16N, R112W) was logged from surface to a total depth of 19,509 ft (5,946 m) and provides excellent geophysical and geological information (formation identification, velocity control, reflection character) from the Tertiary to the Cambrian Gallatin Formation. Although the well does not tie the seismic perfectly because of stratigraphic thickening occurring over the nine mi (14.5 km) projection, it

The formations on the seismic line identified as key reflections (dashed horizons of Figs. 3,5) are

The interpretation of this seismic line (Figs. 3,5) was performed by identdying rock boundaries and

Williams and Dixon J Wyoming Overthrust Belt 21

does provide a relatively undisturbed representation of stratigraphy that could not be found elsewhere along the seismic section.

Problems Unique to the Area

of pitfalls (Tucker and Yorston, 1973). The problems originate from the attempt to take complex thrust- belt geology, which in the real world is three-dimensional, and represent this in a two-dimensional, time-domain display.

Many of the problems encountered in the interpretation of thrustbelt seismic data fall into the realm

Specific velocity problems in the area can be attributed to some or all of the following: 1) Lateral changes in Tertiary rock thickness and composition. 2) Differential mini-thrust movements within the hanging wall of the major thrust. 3) Paleozoic (and Mesozoic) strata thrust over Cretaceous (and Tertiary) sedimentary rocks. 4) Varying shapes and thicknesses of salt diapirs. 5) Overthickening of shale packages in the autochthonous block due to lateral piercement

The counterpart to velocity problems is geometry. Specifically these include (for a 2-D seismic (leading edge of the Darby Thrust).

line): 1) offcline energy contribution from 360 about the line. 2) Diffractions from the numerous faults on (and off) the line. 3) Imaging problems due to steep dips and overturned beds (refer to Ryckman Creek, east limb,

Fig. 2).

CONCLUSIONS There is a Latin phrase worth remembering when the interpretation of a 2-D (and 3-D for that

matter) seismic line in the thrustbelt is about to be undertaken. The expression is caveat emptor, or let the buyer beware. Rather than accepting the subsurface geologic cross section implied from the time section, it should be realized immediately that a depth conversion will oftentimes present a completely different picture. Wells have been drilled in the Overthrust Belt on subthrust velocity pull-ups (looked like an anticline), buried focus effects (looked like an anticline), and ray path dis- tortions (data drop-out zone looked like a fault). Many of the more obvious pitfalls (Tucker and Yorkston, 1973) can be avoided by incorporating the following information in every seismic inter- pretation: structural styles and area trends, surface geology, well information, geologicaVgeophysica1 models, and depth conversion and migration of the seismic data. The result will be a more accurate and correct interpretation.

SEISMIC ACQUISITION AND PROCESSING Special Acquisition Techniques

The eastern part of the line in Figure 2 was recorded with a group interval of 440 ft (134 m) under the assumption of monoclinal dip with no structural complications. The remainder of the line was shot with a 220 ft (67 m) group interval, 48 channel, split spread corfiguration. Single hole patterns of 50 pound (22.7 kg) charges at 140 ft (42.7 m) depths were used at every third station to achieve an eight fold coverage.

employed, the quality of the line is quite good. The line was acquired in 1972 by Amoco; and although no special acquisition procedures were

Special Processing Techniques The processing procedures that generated the stacked section include: datum statics, gain recovery,

instrument dephase, spectral whitening, velocity analysis, surface consistent statics, first break mute, stack, time variant filter and scale. Wave equation migration (finite difference method) was applied to the stacked section using five smoothed velocity functions along the line, which was then filtered and scaled.

ACKNOWLEDGEMENTS

Company in the search for more understanding of regional overthrusts. We also thank the many workers in this area for their suggestions and individual interpretations which were utilized in generalizations.

The authors gratefully acknowledge the cooperation and encouragement of Champlin Petroleum

REFERENCES CITED Angevine, C.L., and Turcotte, D.L., 1983, Oil generation in overthrust belts: American Association

of Petroleum Geologists Bull., v. 67, p. 235-241. Burchfiel, B.C., and Davis, G.A., 1975, Nature and controls of Cordilleran orogenesis, western United

States; extensions of an earlier synthesis: American Journal of Science, v. 275A, p. 363-396. Coney, PJ., 1973, Plate tectonics of marginal foreland thrust-fold belts: Geology, v. 1, p. 131-134. Dickinson, W.R., 1976, Sedimentary basins developed during evolution of Mesozoic-Cenozoic arc-

trench system in westem North America: Canadian Journal of Earth Science, v. 13, p. 1268-1287. Dixon, J.S., 1982, Regional structural synthesis, Wyoming Salient of western Overthrust Belt:

American Association of Petroleum Geologists Bull., v. 66, p. 1560-1580. Jordan, T.E., 1981, Thrust loads and foreland basin evolution, Cretaceous, western United States:

American Association of Petroleum Geologists Bull., v. 65, p. 2506-2520. Lowell, J.D., 1977, Understanding origin for thrust-fold belts with application to the Idaho-Wyoming

belt: Wyoming Geological Assoc. 29th Ann. Field Conf. Guidebook, p. 449-455. Misch, P., 1960, Regional structural reconnaissance in central-northeast Nevada and some adjacent

areas; observations and interpretations: Intermountain Assoc. Petroleum Geologists and Eastern Nevada Geological Society, 1 1 th Ann. Field Conf. Guidebook, p. 17-42.

Petroleum Exploration, 1981, The Overthrust Belt - 1981: publ. by Petroleum Information Corp., 251 p.

Powers, R.B., 1983, Petroleum potential of wilderness lands in Wyoming-UtaheIdaho thrust belt: U.S. Geological Survey Circular 902-A-P, Betty M. Miller, editor, p. N1-N14.

Tisoncik, D.D. (in press), Regional lithostratigraphy of the Phosphoria Formation, western Overthrust: Rocky Mountain Assoc. Geologists, Rocky Mountain Source Rock Symposium.

Tucker, P.M., and Yorston, H.J., 1973, Piefalls in Seismic Integretatiun: Society of Exploration-Gee physicists, Monograph Series No. 2, Tulsa, Okla., p. 1 ff.

Warner, M.A., 1980, Source and time of generation of hydrocarbons in Fossil basin, western Wyoming thrust belt (abs.): American Association of Petroleum Geologists Bull., v. 64, p. 800.

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Williams and Dixon Wyoming Overthrust Belt 22

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Bonneville County, Idaho Seis-Port Mark VI Heliportable drills working in northern overthrust belt on proprietary program for Anschutz Corporation.

Photo by McAllister of Denver for Seis-Port Expl .

Paradox Basin Parriott Mesa, Fold & Fault Belt, Seis-Port Mark VI Drills drilling 80' shot holes. A typical Heliportable rig weighs 1400 lbs and

can be moved in three lifts consisting of the engine, compression and skidmast unit by medium lift helicopters such as the Lama 315 B. Photo by McAllister of Denuerfor Seis-Port Expl.